Selective deposition response to aeolian–ﬂuvial … · braided stream channels. Both aeolian and ﬂuvial sediment supplies regulate sediment transport and deposition in such

Nat. Hazards Earth Syst. Sci., 15, 1955–1962, 2015

www.nat-hazards-earth-syst-sci.net/15/1955/2015/

doi:10.5194/nhess-15-1955-2015

© Author(s) 2015. CC Attribution 3.0 License.

Selective deposition response to aeolian–fluvial sediment supply

in the desert braided channel of the upper Yellow River, China

H. Wang1,a, X. Jia1, Y. Li1, and W. Peng1

1Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering Institute,

Chinese Academy of Sciences, Gansu Province 730000, Chinaanow at: Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, 260

Donggang West Road, Lanzhou, Gansu Province, 730000, China

Correspondence to: H. Wang ([email protected])

Received: 23 December 2014 – Published in Nat. Hazards Earth Syst. Sci. Discuss.: 10 February 2015

Revised: 7 July 2015 – Accepted: 19 August 2015 – Published: 2 September 2015

Abstract. Rivers flow across aeolian dunes and develop

braided stream channels. Both aeolian and fluvial sediment

supplies regulate sediment transport and deposition in such

cross-dune braided rivers. Here we show a significant se-

lective deposition in response to both aeolian and fluvial

sediment supplies in the Ulan Buh desert braided channel.

The Ulan Buh desert is the main coarse sediment source

for this desert braided channel, and the mean percentage

of the coarser ( > 0.08 mm) grains on the aeolian dunes

surface is 95.34 %. The lateral selective deposition process

is developed by the interaction between the flows and the

aeolian–fluvial sediment supplies, causing the coarser sedi-

ments (> 0.08 mm) from aeolian sand supply and bank ero-

sion to accumulate in the channel centre and the finer flu-

vial sediments (< 0.08 mm) to be deposited on the bar and

floodplain surfaces, forming a coarser-grained thalweg bed

bounded by finer-grained floodplain surfaces. This lateral se-

lective deposition reduces the downstream sediment trans-

port and is a primary reason for the formation of an “above-

ground” river in the braided reach of the upper Yellow River

in response to aeolian and fluvial sediment supplies.

1 Introduction

In nature, some rivers flow across active dune fields and

become shifted and braided, developing many channel bars

and large-area floodplains (Smith and Smith, 1984; Ta et al.,

2008). Smith and Smith (1984) indicated that abrupt addi-

tion of aeolian sands to rivers can lead to a 40-fold increase

in bed load, a 5-fold increase in width and a 10-fold increase

in width / depth ratio in a small desert river (William River,

Canada), and is a primary mechanism for the development

of such a braided stream channel. Actually, large cross-dune

rivers are also fed by high rates of upstream suspended sedi-

ment supplies (Ta et al., 2008, 2011). However, it is still less

clear how these two aeolian and fluvial sediment supplies can

be regulated to influence sediment transport and deposition in

such a cross-dune desert river.

In gravel-bed rivers, the channel bed is composed of

two components: sand (< 2 mm) and gravel (> 2 mm).

Wilcock (1998) indicated that the sand supply (< 2 mm) has

a great impact on gravel transport and size change of bed

sediment in gravel-bed rivers. If the sand supply is decreased

or increased from upstream, the channel bed of gravel-bed

rivers will become coarser or finer, respectively (Dietrich

and Smith, 1983; Ferguson et al., 1989, 1996; Parker and

Sutherland, 1990; Hoey and Ferguson, 1994; Pizzuto, 1995;

Wilcock, 1998; Gasparini et al., 1999; Lisle et al., 2000;

Wilcock and Kenworthy, 2002; Singer, 2008). Since cross-

dune rivers with a low gradient are sand-bed rivers, their bed

sediment sizes are actually less than 2 mm and are in a state

of fully mobilized transport, which makes differences in the

threshold of motion between coarser and finer grains rela-

tively unimportant and give all sizes of sediments equal mo-

bility (Frings, 2008). Church (2006) indicated that bedloads

are relatively coarse and make up the beds and lower banks of

the river channel, but suspended loads are finer and may be an

important constituent of the upper banks. Ta et al. (2011) also

showed that mid-channel bars are distinguished with a finer

Published by Copernicus Publications on behalf of the European Geosciences Union.

1956 H. Wang et al.: Aeolian–fluvial sediment supply in the desert braided channel of the upper Yellow River

Figure 1. Schematic illustration of the Ulan Buh desert braided

channel of the Yellow River, China. (a) The Yellow River flows

through the Ulan Buh Desert and the Kubuqi Desert, the braided

channel is from Wuhai to Sanhuhekou, and the meandering channel

is from Sanhuhekou to Toudaoguai; (b) the studied desert channel;

(c) the Yellow River and Yangtze River in China.

surface layer (< 0.08 mm) developing on a subsurface layer

(> 0.08 mm) in the sand-bed braided and meandering rivers

in the upper Yellow River. These results suggest that although

the sand-bed channel shows an equal threshold of motion be-

tween coarser and finer grains, it can actually transport and

deposit sediments selectively rather than uniformly (Frings,

2008; Wright and Parker, 2005; Ta et al., 2011). Because ae-

olian sands generally are coarser than river-suspended sed-

iments in grain size (Ta et al., 2011), we propose that ae-

olian and fluvial sediment supplies may be transported as

bedloads and suspended loads, respectively, leading to se-

lective deposition in different zones to form size segregation

in braided channels. However, until now, there have been no

field and flume data to support this hypothesis. Nonetheless,

understanding the size segregation mechanism is important

for predicting sediment transport and deposition and channel

change in braided channels.

Here we present field evidence of selective deposition in

the Ulan Buh desert braided channel of the Yellow River,

China, which responds to finer (< 0.08 mm) suspended sedi-

ment supply from the upstream and coarser (> 0.08 mm) ae-

olian sand supply from the Ulan Buh desert (Ta et al., 2011).

Our main objective is to clarify the lateral selective deposi-

tion mechanism in response to aeolian and fluvial sediment

supplies in a large, low-gradient, and sand-bed braided river,

and to shed some light on what the effect of this selective

deposition is on the channel morphologies in braided rivers.

Figure 2. Changes in annual flow and sediment discharges moni-

tored in the Shizuishan gauge and the Bayangaole gauge (data come

from the YRCC) from 1955 to 2007 in the Yellow River.

2 Study area

To clarify the size segregation mechanism in response to the

aeolian and fluvial sediment supply, we chose the 60 km long

section of the Ulan Buh desert channel of the Yellow River,

China (Fig. 1). This desert channel is a braided channel with

an average gradient of 0.00028 and has no confluence with

tributaries. According to long-term observations (data are

from 1955 to 2007) by the Yellow River Conservation Com-

mission (YRCC), the finer sediment supply (< 0.08 mm)

from the upstream is about (1.23 ± 0.8) × 108 t yr−1 but

about (1.21 ± 0.8) × 108 t yr−1 of the suspended sediment

is transported out of the desert channel (Fig. 2). The re-

sults of variance analysis showed that there was no signifi-

cant variation in the finer sediment load between two gauges

(P > 0.05).

The Ulan Buh desert is located in the northwest of the Yel-

low River and topography is gradually tilted from northwest

to southeast. The dune heights are 7–20 m and the mean per-

centage of the coarser (> 0.08 mm) and finer (< 0.08 mm)

grains on the aeolian dunes surface from the Ulan Buh desert

are 95.34 and 4.66 %, respectively. Under the W and NW-

dominant strong winds in the region, the strong windblown

sand/dust activity transports a large amount of the coarse

sands entering the desert channel and causes dunes to move

forward. Ta et al. (2008) estimated that the coarser sedi-

ment (> 0.08 mm) input from the local Ulan Buh desert re-

gion is about 0.2 × 108 t yr−1 on long-term average. Based on

coastal dunes mobile monitoring results, He et al. (2012) sug-

gested that the dunes move forward a distance of 8.19 m yr−1

towards the river channel, mainly occurring in March to May.

Thus, the Ulan Buh desert is the main coarse sediment source

for the study area of this desert channel.

Due to the increase of coarse sediment loads and the reduc-

tion of annual flow discharge upstream, this desert channel

has shown aggradation and a significant decrease in bank-

full discharge during the last 30 years (Wu and Li, 2011) and

is characterized by lateral shift, leading to a wide distribu-

tion of river bars and floodplains (Ta et al., 2008). Therefore,

Nat. Hazards Earth Syst. Sci., 15, 1955–1962, 2015 www.nat-hazards-earth-syst-sci.net/15/1955/2015/

H. Wang et al.: Aeolian–fluvial sediment supply in the desert braided channel of the upper Yellow River 1957

this desert channel provides an unusual opportunity to study

size segregation in response to the aeolian and fluvial sedi-

ment supply and the development of large-area floodplains

in large braided rivers.

3 Methods

In the Ulan Buh desert reach, the Yellow River Engineering

and Management Bureau of the Inner Mongolia Autonomous

Region (YREMB) installed 23 cross sections and surveyed

them in April and October every year for about 45 years

(1966–present). Along these cross sections, sediment sam-

ples on the main channel beds and bars or floodplain sur-

faces have also been collected and their grain size distribu-

tions have been analysed. This study provided long-term data

on changes in the channel lateral shift and the grain size of

bed sediments. However, there have been no reports on the

lateral size segregation in response to the aeolian and flu-

vial sediment supplies and the related lateral channel shifts.

Here we choose 12 wider cross sections (C8, C10, C12, C14–

C22) in the desert channel (Fig. 1b), which is fed by aeolian

and fluvial sediment supply and shows high rates of lateral

channel shifts. We complied 45-year (1966–2011) monitor-

ing data of these 12 cross profiles and their related grain size

distributions of bed sediments (C15, C17, C19, and C21 were

not included because their bed sediments were not collected)

from the YREMB to study the lateral size segregation of the

channel bed sediments and the lateral channel shift in the

Ulan Buh desert braided reach. We chose 16 channel bars

(N1–N16) in the desert channel and took 3 m-deep core sed-

iment samples from them (Fig. 1b). These sediment cores

were cut and separated to obtain 4 cm column samples, and

dried and sieved to analyse the vertical size distributions. We

collected suspended sediment samples at three vertical pro-

files in the main channel, with a 30 cm height interval at the

1.5 m-thick near-bed layer, and a 50 cm height interval at the

upper layer along the cross section in the Sanhuhekou gauge

station (Fig. 1a) to analyse the vertical size distribution of

suspended sediments. These data were used to analyse the se-

lective transport of all sizes of transported sediments during

the passage of a flood. We also choose 11 high-flow floods

(> 3500 m3 s−1) from 1955 to 2012 (monitored by Shizuis-

han and Bayangaole gauges), which provide strong evidence

to confirm that the suspended sediment loads in the desert

braided channel should be attributed to upstream sediment

supplies rather than bed coarser sediments from aeolian sup-

plies or bank erosions.

4 Results

The long-term monitoring data of 12 wider cross sections

in the Ulan Buh desert reach of the Yellow River indicated

that the main channel beds show coarser-grained sediments

(> 0.08 mm) but the bar or floodplain surfaces show finer-

Figure 3. The median sizes of the thalweg bed sediments and the

bar or floodplain surface sediments (BFS) in the Ulan Buh desert

braided channel from 1975 to 2005.

grained sediments (< 0.08 mm) (Fig. 3), suggesting a lat-

eral size selective deposition in response to coarser aeolian

(> 0.08 mm) and finer fluvial (< 0.08 mm) sediment sup-

plies. This lateral sediment deposition is also found to be

accompanied by vertical size selective deposition, leading

to finer surface layers (< 0.08 mm) developing over coarser

subsurface layers (> 0.08 mm) in the 16 channel bars in the

desert braided channel (Fig. 4). These finer surface layers are

thinner (50–120 cm) and are sometimes lacking; for example,

there are no finer surface layers in the five channel bars (N3,

N8, N9, N13, and N14) in the braided channel. Our observa-

tions also indicated that this selective deposition is primarily

accommodated through selective transport, during which the

< 0.08 mm size fraction is found to be transported as sus-

pended loads, but the > 0.08 mm size fraction is transported

primarily as bedloads (Fig. 5) at flow discharge conditions

(1000–3000 m3 s−1) (Fig. 6). These results support our hy-

pothesis and indicate that the aeolian and fluvial sediment

supplies cannot be mixed effectively in the braided channel

and therefore tend to be primarily transported separately as

bedloads and suspended loads, respectively, leading to se-

lective deposition in different zones and forming a coarse-

grained main channel bed bounded by fine-grained bar or

floodplain surfaces.

This lateral size segregation develops through the lateral

channel shift in the braided channel. When the channel mi-

grates towards the aeolian sand-covered bank(ACB), the ae-

olian dunes are eroded and side bars develop on the opposite

banks, or mid-channel bars develop between the channels;

but if the channel moves leaving from the ACB, the sand

bars form between the main channel and the ACB, which in

turn block the wind-blown sands entering the stream chan-

nel (Fig. 7). This lateral channel shift causes the coarse aeo-

lian sands to accumulate around the channel centre and drives

the suspended sediments to be deposited on the surfaces of

sand bars. During this lateral shift, the main thalweg bed rose

www.nat-hazards-earth-syst-sci.net/15/1955/2015/ Nat. Hazards Earth Syst. Sci., 15, 1955–1962, 2015


Figure 4. Vertical distributions of portions of the two-fraction sediments (> 0.08 mm and < 0.08 mm) and the related median sizes in 16

sediment cores (N1–N16, see in Fig. 1b) in the channel bars in the Ulan Buh desert braided channel. Olive-green lines depict curves of the

portions of the > 0.08 mm sediments, dark cyan lines depict curves of the portions of the < 0.08 mm sediments and blue circles depict curves

of the median sizes of sediments.

about 1.33 m on average in the range of 0.169 to 2.295 m dur-

ing the forty-five years (Fig. 8).

Although the channel shows high rates of lateral shifts in

response to high-rate flow discharges, the suspended sedi-

ment concentrations in the desert reach primarily vary in

response to sediment supplies from upstream but were sur-

prisingly low (6.48–6.88) in response to the flow discharge

peaks near 5000 m3 s−1 (Figs. 9 and 10). Linear fitting us-

ing a least squares method indicates that when the flow dis-

charge ranges from 3500 to 5210 m3 s−1, the suspended sed-

iment load in the desert braided channel can actually be regu-

lated by the upstream supplies rather than the flow discharges

(Fig. 11). This result indicates that it may be hard for the

> 0.08 mm bed sediments to be suspended from the bed as

suspended sediment loads at such a low gradient (0.00025)

and sand-bed river during the passage of a flood. The re-

sult further indicates that the portion of the finer (< 0.08 mm)

sediments from the sand bar or floodplain can be transported

downstream as suspended sediment loads, which make the

suspended sediment load at the Bayangaole gauging station

greater than that at Shizuishan gauging station. Although the

mean percentage of the finer (< 0.08 mm) grains from the

sand bar or floodplain are 30.71 %, only some of the finer

(< 0.08 mm) surface sediments can be transported down-

stream during the flood events. Due to the higher transport

capacity of the river for the finer (< 0.08 mm) sediments, the

beginning of the flow discharge increases in a flood event,

and the limited finer (< 0.08 mm) surface sediments form a

higher peak in suspended sediment concentrations. With the

flow discharge continuously increasing and the insufficient

supply of fine sediment particles, the peak value for the dis-

charge and sediment differed in time.

5 Discussions and conclusions

Our results suggest that the significant lateral size segre-

gation can be produced in response to aeolian and fluvial

sediment supplies in our studied braided channel. Aeolian

sand supply and bank erosion provide enough available bed-

loads which contribute to the primary bed sediments and con-



Figure 5. Vertical profiles of proportions of the two-fraction sediments (> 0.08 mm and < 0.08 mm) in the cross section of the Sanhuhekou

gauge during the passage of a flood with a flow discharge from 1000 to 3200 m3 s−1 in 2012. (a) 540 m distance from the starting point of

the cross section in the left bank, (b) 620 m distance from the starting point of the cross section in the left bank, (c) 700 m distance from the

starting point of the cross section in the left bank.

Figure 6. Flow discharges monitored in three gauges (Shizuishan,

Sanhuhekou and Toudaoguai) in the braided and meandering chan-

nels during the passage of a flood in 2012.

trol the development of the braided channel. Although flu-

vial sediment supplies from the upstream are larger in quan-

tity than the aeolian sand supplies, they actually are wash

loads; this shows the well-known phenomenon of “the more

it come, the more it goes” (Wu et al., 2008), and therefore

cannot be deposited in the main channel bed but can be de-

posited in slack water on bar tops and overbank during floods

(Church, 2006). Since the braided channel is unstable and

shifts laterally, the bar or floodplain tends to be eroded and

its finer surface sediments can be transported downstream as

suspended loads, but the coarser subsurface sediments show

local erosion and deposition processes and thereby should

be of major importance in determining braided channel mor-

phology (Church, 2006).

Some studies have shown that the helical secondary

flow (HSF) develops by the skewing of cross-stream vortic-

ity into a long-stream direction, carrying faster surface water

towards the outer bank and slower bed water towards the in-

ner bank in a braided or meandering channel (Dietrich and

Smith, 1983; Thorne et al., 1985). The HSF erodes the outer

bank and fills the inner bank, leading to an asymmetrical

cross section and a lateral channel shift. Because the cross

profile of the stream channel is roughly a parabola and be-

cause the downslope gravity component of sand grains is a

body force and the fluid drag component is a surface force,

increased grain size of sediments caused an increase in the

downslope gravity component of grains greater than in the

fluid drag and accelerated the accumulation of coarse sedi-

ments in the channel centre (Wilson, 1973).

Although the HSF was not examined in our studies, it

plays an essential role in the development of the lateral size

segregation in response to aeolian and fluvial sediment sup-

plies. As wind-blown sands move downwind and meet the

stream channel, the wind cannot carry sands continuously

in the water, and therefore tend to deposit aeolian sands on

channel sides to form aeolian dunes in the stream banks.

In these conditions, if the HSF drives the channel towards

the aeolian sand-covered bank, the aeolian dunes are eroded

and the aeolian sands can creep along the outer bed slope

towards the channel centre under the influence of the fluid

drag and the gravity component; but on the inner bed slope

of the opposite bank, the fluid drag and the gravity compo-

nent are in a different direction and their balance will de-

termine and separate the coarser and finer sediment deposi-

tion zones, causing the coarser sediments to accumulate in

the channel bottom and the finer suspended sediments to de-

posit on the bar platform surfaces. However, if the HSF drives

the main channel away from the aeolian sand-covered bank,

the suspended sediments in turn tend to be transported to-

wards the aeolian sand-covered bank and to be deposited to

form new bars or to expand old floodplains. This expanded

floodplain thereby separates the main channel and the aeo-

lian sand-covered bank and blocks wind-blown sands enter-

ing the main channel. This channel shifts continuously back

and forth, causing coarser aeolian sands to accumulate in the

channel centre and causing finer sediments to be deposited

on the bar or floodplain surfaces on the channel sides; this is

a primary mechanism for the long-term size segregation and

channel change in the braided channel.



Figure 7. Changes of the cross section of C8 from 1966 to 2011. ACB is the aeolian sand-covered bank, STA is the channel shift towards the

ACB and SLA is the channel shift leaving from the ACB. (a), (c), (e), (g) and (h) indicate that when the channel shifts toward the ACB, the

sand dunes are eroded and the aeolian sands are transported towards the channel centre and the point bars are then formed on the opposite

bank. (b), (d), (f), and (i) show that as the channel moves away from the ACB, the point bars are formed between the main channel and the

ACB, which in turn block wind-blown sand entering into the channel.

Figure 8. Lateral channel shifts (b) and the rising thalweg beds (a)

monitored from 1965 to 2012.

Our results suggest that the braided channel is stronger in

the lateral size segregation than in the longitudinal size segre-

gation, which is supported by evidence that the cross profile

adjusts to changing conditions much more rapidly than the

long profile, as suggested by Wilson (1973). Because aeolian

sand supply originates from river banks, which differs from

fluvial sediment supply through stream channels, the tradi-

tional downstream fining of bed sediments shows no signifi-

cance in the aeolian sand-fed braided channel. Frings (2008)

argued that the size segregation primarily resulted from the

presence of suspended load transport in combination with

the effects of dune and bend sorting in large sand-bedded

rivers, making selective transport and deposition of differ-

ent size sediments and producing downstream fining of bed

sediments. Although he mainly focused on the longitudinal

size segregation in large sand-bedded rivers, he also realized

the importance of sediment addition and extraction, and the

influence of changes of overbank sedimentation on the bed

sizes in large sand-bedded rivers.

We propose that in rivers which flow through aeolian

dunes, increased bedload supply, as a result of increased ae-

olian processes or accelerated bank erosions, may cause the

channel to be filled with coarser sediments and consequently

become a shallow and shifting braided channel. If accelerated

erosion leads to increased finer sediment supply from up-

stream, the HSF will respond by transporting the suspended

sediment towards the stream sides and by accelerating the de-

position of the finer sediments on the point bar or floodplain

surfaces. Since the HSF causes the lateral selective deposi-

tion and consequently reduces downstream sediment trans-

port, the braided channel will show aggrading in response to

both aeolian and fluvial sediment supplies. This mechanism

may explain the formation of the “above-ground river” in the

braided reach of the upper Yellow River (Ta et al., 2011).

Because the HSF may regulate the lateral size segregation in



Figure 9. Daily suspended sediment loads and flow discharges of three flow peaks (> 4000 m3 s−1) flowing through the Ulan Buh desert

braided channel. (a) 5050 m3 s−1 flow discharge peak on 31 July 1964; (b) 4990 m3 s−1 flow discharge peak on 16 September 1967;

(c) 5210 m3 s−1 flow discharge peak on 21 September 1981. Light grey and grey lines depict curves of flow discharges monitored in Shizuis-

han and Bayangaole gauges, respectively; olive-green and blue lines depict curves of suspended sediment loads monitored in Shizuishan and

Bayangaole gauges, respectively.

Figure 10. Daily suspended sediment loads and flow discharges of eight flow peaks (3500–4000 m3 s−1) flowing through the Ulan Buh

desert braided channel. (a) 3990 m3 s−1 flow discharge peak on 20 July 1955; (b) 3550 m3 s−1 flow discharge peak on 4 October 1963;

(c) 3780 m3 s−1 flow discharge peak on 23 September 1968; (d) 3790 m3 s−1 flow discharge peak on 13 September 1976; (e) 3660 m3 s−1

flow discharge peak on 19 September 1978; (f) 3630 m3 s−1 flow discharge peak on 24 August 1983; (g) 3790 m3 s−1 flow discharge peak

on 5 August 1984. Light grey and grey lines are curves of flow discharges monitored in Shizuishan and Bayangaole gauges, respectively;

olive and blue lines are curves of suspended sediment loads monitored in Shizuishan and Bayangaole gauges, respectively.



Figure 11. Relation of suspended sediment loads monitored in the

Shizuishan gauge (C0) and the Bayangaole gauge (C).

braided and meandering rivers, HSF-size segregation interac-

tion is a topic that deserves further consideration and study.

Acknowledgements. This work was supported by the National

Basic Research Program of China (no. 2011CB403302) and the

One-Hundred Talents Project of CAS “Desert Surface Processes

and Mechanisms” and the Natural Science Foundation of China

(No. 41171011).

Edited by: T. Glade

Reviewed by: S. J. Wang and another anonymous referee

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Selective deposition response to aeolian–ﬂuvial … · braided stream channels. Both aeolian and ﬂuvial sediment supplies regulate sediment transport and deposition in such